An electronic control unit for a vehicle, the electronic control unit comprising a processor comprising: a processor core; storage, the storage storing data comprising instructions for the processor core; a tamper-resistant hardware security module which is coupled to the storage for reading and writing; and an external interface; the electronic control unit further comprising further storage connected to the processor through the external interface and containing further data; in which the hardware security module is arranged to cause a determination whether the data in the storage has been tampered with and, on a determination that the data has been tampered with, to cause the further data to be loaded into the storage from the further storage over the external interface. Other apparatus and methods for improving the security of electronic control circuits are disclosed.